The invention is directed to improvements in a hydraulic dual-circuit brake system with an anti-skid system and traction control for motor vehicles, in particular passenger vehicles.
In a known dual-circuit brake system of this type (German Patent Document 38 16 073 A) the outlet channels of the four-channel hydraulic unit for the wheel brake cylinders of the driven wheels are assigned to one brake circuit, and the outlet channels for the wheel brake cylinders of the non-driven wheels are assigned to the other brake circuit. The various control valves in the outlet channel, for controlling a brake pressure as a function of wheel slip, are connected in pairs per brake circuit to the associated brake circuit outlets of the master brake cylinder via an inlet channel of the hydraulic unit and one connecting line each. Also connected to the same inlet channel is the outlet of the pumping element, operative in the applicable brake circuit, of the return pump. The switching means for connecting the return pump to the master cylinder or to the brake fluid tank have a reversing valve, which is disposed in the connecting line between the hydraulic unit inlet channel communicating with the brake circuit of the driven wheels and the one brake circuit outlet of the master brake cylinder. The connecting means for connecting the brake fluid tank to the return pump has a hydraulically controllable 2/2-way valve, disposed in the additional hydraulic unit, and a third pumping element, acting as a pre-charging pump, in the return pump; these are disposed in succession between the brake fluid tank the inlet to the pumping element that is associated with the brake circuit of the driven wheels. The control inlet of the 2/2-way valve is connected to one brake circuit outlet of the master brake cylinder. When there is control pressure at the control inlet, the multi-position valve is switched over to its blocking position. In traction control the reversing valve is reversed and now disconnects the brake circuit from the brake circuit output of the master brake cylinder, and connects the brake circuit to the brake circuit outlet of the master brake cylinder via a pressure-controlled valve. The precharging pump acting as a low-pressure pump now supplies the following pumping element, embodied as a high-pressure pump, of the return pump, which generates a high braking pressure that is present at the inlet to the control valve associated with the driven wheels. This brake pressure is fed via the control valves into the wheel brake cylinders of the driven wheels, which are slowed down as a result. If only one driven wheel is spinning, then the control valve associated with the outlet channel for the wheel brake cylinder of the nonspinning driven wheel is moved to its middle position, so that no brake pressure can be built up in the wheel brake cylinder of the driven wheel that is not spinning. By pressure modulation in the wheel brake cylinder associated with the slipping driven wheel, which modulation is effected by the control valve associated with that cylinder, a brake pressure that is adequate for slowing down the slipping driven wheel is built up. Once there is no longer a loss of traction, the control valve is moved to its terminal position by maximum current excitation; in this position, the outlet channels for both driven wheels communicate with the inlet of the associated pumping element of the return pump. Brake fluid is now pumped out of the wheel brake cylinder by the pumping element and returned to the brake fluid tank via the pressure-controlled valve. The brake pressure in the wheel brake cylinder is reduced fully once again.
Because of the pressure-controlled valve located in the return pumping circuit, the time between reversing the control valve to a terminal position until complete relief of the wheel brake cylinders of the driven wheels is relatively long. In normal traction control (ASR) operation this is no problem. However, if the brake pedal is actuated during traction control, then the brake system must terminate the traction control operation very quickly, which requires the fastest possible pressure relief of the driven wheel brake cylinders acted upon by brake pressure.